Cambodia's war crimes tribunal set free a former leader of the Khmer Rouge on Sunday, upholding a decision that has outraged survivors seeking an explanation of the mass killings committed more than 30 years ago.

Ieng Thirith, 80, who has been declared mentally unfit for trial, was driven out of the UN-backed tribunal's compound by family members. She made no comment to reporters.

The Sorbonne-educated Shakespeare scholar served as social affairs minister during the Khmer Rouge's rule from 1975-79, during which an estimated 1.7 million people died of execution, medical neglect, overwork and starvation.

The tribunal initially announced its decision to free Ieng Thirith on Thursday, saying medical experts had determined there was no prospect for her to be tried due to a degenerative mental illness that was probably Alzheimer's disease.

Prosecutors then delayed her release by filing an appeal demanding that conditions be set to restrict her freedom.

On Sunday,, the tribunal's supreme court said it had accepted the appeal, which is expected to be heard later this month. In the meantime, it set three provisional conditions on her movement.

The tribunal said Ieng Thirith must inform the court of her address, must turn in her passport and cannot leave the country, and must report to the court whenever it summons her.

Ieng Thirith was the Khmer Rouge's highest-ranking woman and also a sister-in-law of the group's top leader, Pol Pot, who died in 1998.

She is accused of involvement in the "planning, direction, co-ordination and ordering of widespread purges," and was charged with crimes against humanity, genocide, homicide and torture.

Three other senior Khmer Rouge leaders are on trial, including her husband, 86-year-old Ieng Sary, the regime's former foreign minister; 85-year-old Nuon Chea, its chief ideologist and second-in-command; and 80-year-old Khieu Samphan, a former head of state.

The tribunal said earlier that Ieng Thirith's release did not mean the charges against her were being withdrawn and was not a finding of guilt or innocence. It plans to consult annually with experts to see whether future medical advances could render her fit for trial, although that is considered unlikely given her age and frailty.

Survivors of the Khmer Rouge called Ieng Thirith's release shocking and unjust. They said they had waited decades for justice and found it hard to feel compassion for her suffering.

"It is difficult for victims and indeed, all Cambodians, to accept the especially vigorous enforcement of Ieng Thirith's rights taking place at the [tribunal]," said Youk Chhang, director of the Documentation Center of Cambodia, a group that researches Khmer Rouge atrocities.

In a statement on Sunday,, he noted the irony of Ieng Thirith receiving "world class health care." As social affairs minister she was "personally and directly involved in denying Cambodians even the most basic health care during the regime's years in power," he said.

The tribunal began in 2006 – nearly three decades after the fall of the Khmer Rouge – following years of wrangling between Cambodia and the UN. The lengthy delays have been costly and raised fears that the former leaders could die before their verdicts come.